9.10. Chaining exception handlers

In some situations there can be several different sources
of a particular exception. For example:

Angel
uses an undefined instruction to implement breakpoints. However, undefined
instruction exceptions also occur when a coprocessor instruction
is executed, and no coprocessor is present.

Angel uses a SWI for various purposes, including
getting into supervisor mode from user mode and supporting semihosting
requests. However, an RTOS or an application may also wish to implement
some SWIs.

In such situations there are two approaches that can be taken
to extend the exception handling code. These are described below.